Anti-skid brake control system with a feature for deriving target wheel speed

ABSTRACT

An anti-skid brake control system controls braking pressure through one or more skid cycles in which braking pressure is increased and decreased cyclically so as to prevent the vehicle wheels from locking. The system derives a braking pressure decreasing criterion on the basis of variations in wheel speed over the preceding skid cycle. In order to derive the pressure decreasing criterion, an initial wheel speed value is latched and stored when the system first becomes active. At the same time, measurement of elapsed time starts. Subsequently, the wheel speed at the beginning of each skid cycle is latched and compared to the initial wheel speed. On the basis of the difference between the latched values, and the elapsed time, a wheel deceleration rate is derived. The pressure decreasing criterion is then derived on the basis of the wheel speed latched at the beginning of the current skid cycle and the wheel speed deceleration rate and elapsed time since the beginning of the current skid cycle.

BACKGROUND OF THE INVENTION

The present invention relates generally to an anti-skid brake controlsystem for an automotive brake system which optimizes vehicular brakingcharacteristics and minimizes braking distance. More specifically, theinvention relates to measurement of vehicle speed deceleration for usein deriving a target wheel speed relative to the vehicle speed.

As is well known, optimum braking characteristics are obtained whenbraking pressure or force can be so adjusted that the peripheral speedof the wheels during braking is held to a given ratio, e.g. about 80% to85%, of the vehicle speed. This practice is believed to be particularlyeffective when road conditions and other factors are taken intoconsideration. On the other hand, if the wheel speed/vehicle speed ratiois maintained higher than the above-mentioned optimal ratio, e.g., 80%to 85%, braking distance may be prolong due to a lack of brakingpressure. On the other hand, if the braking pressure is so adjusted asto maintain the wheel speed/vehicle speed ratio with respect the vehiclespeed less than the aforementioned optimal ratio, the vehicle wheels maylock and skid, resulting in an unnecessarily long braking distance dueto reduced traction. In practice, it is very difficult to preciselyadjust the braking pressure so that the wheel speed is held to the givenoptimal ratio to the vehicle speed.

In the practical anti-skid brake control operation, braking pressure inone or more wheel cylinders is adjusted by cyclically increasing anddecreasing the braking pressure in the wheel cylinder. The anti-skidcontrol system generally decreases braking pressure when the wheeldeceleration value becomes less than a given deceleration threshold,which is so chosen as to prevent the wheel from skidding, and increasesbraking pressure when the wheel acceleration value is greater than agiven acceleration threshold. In this conventional anti-skid brakecontrol procedure, wheel speed does not remain at an optimalrelationship to the vehicle speed for a satisfactorily long period oftime.

U.S. Pat. No. 3,637,264, issued on Jan. 25, 1972 to Leiber et aldiscloses an Antilocking Control for Pressure Actuated Brakes. Thepressure of the brake-actuating fluid in an antilocking brake controlsystem is varied by pulsing the control valve or valves for durationswhich are varied to be greater or lesser than the period of thatlimiting frequency above which the brake system cannot respond. In theformer case, a rapid increase in fluid pressure or a rapid decrease influid pressure occurs, whereas in the latter case, a less rapid averageor net increase or decrease occurs in the fluid pressure to which thebrake system responds. These conditions are controlled in dependence onthe rotational behavior of the vehicle wheel or wheels and moreespecially in dependence or predetermined changes in angular velocity ofthe wheel. Moreover, either variation in pulse duration at a fixedfrequency or variation in frequency at a fixed pulse duration may beeffected during high-frequency pulsing so as further to alter the netincrease or decrease in fluid pressure. This further alternation iseffected as a function of time from the beginning of the high-frequencypulsing.

In addition, Published Japanese Patent Application (Tokkai) Showa No.51-89096, published on Aug. 4, 1976 discloses a system similar to theabove. The fluid pressure in the wheel cylinder is increased in astepwise manner. Duration of the increase of the fluid pressure isadjusted in accordance with the rate of increase of the fluid pressurein one or more preceding steps.

Another approach for deriving the target wheel speed has been disclosedin U.S. Pat. No. 4,430,714, issued on Feb. 7, 1984, to the commoninventor and commonly assigned. In the shown system, wheel speed issampled and held every time wheel acceleration drops below apredetermined deceleration threshold. The intervals between samplings ofthe wheel speed value are measured. On the basis of the differencebetween the wheel speed in the current skid cycle and that measured inthe immediately preceding skid cycle and the measured interval betweensamplings of the wheel speed in the current and immediately precedingcycles, wheel speed deceleration rate is derived. The derived wheeldeceleration rate is used in deriving the target wheel speed for thenext skid cycle of brake control operation.

This conventional technique cannot derive the target wheel speed withsufficient precision. In particular, variations in road surface frictionduring anti-skid brake control make it difficult to hold the brakepressure near the lock pressure at which the vehicle wheels start toskid. For instance, if road surface friction decreases, thus decreasingthe lock pressure, the wheels may be induced to skid if the target wheelspeed derived by way of the above technique is adhered to. On the otherhand, if the road friction increases, the wheel speed deceleration ratederived by way of the above procedure may become unnecessarily great,resulting in significant fluctuations in skid control.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide an anti-skidbrake control system which reduces fluctuations in wheel speeddeceleration rate between successive skid cycles.

Another and more specific object of the invention is to provide animproved procedure for deriving a target wheel speed which fluctuatesless than values derived by conventional techniques.

In order to accomplish the aforementioned and other objects, ananti-skid brake control system, according to the present invention, aninitial wheel speed value is latched and stored when the system becomesactive. At the same time, measurement of elapsed time starts.Subsequently the wheel speed at the beginning of each skid cycle islatched and compared to the initial wheel speed. On the basis of thedifference between the value latched at the beginning of each skid cycleand the initial wheel speed value and of elapsed time at the beginningof the current skid cycle, wheel deceleration rate is derived. A targetwheel speed is then derived on the basis of the wheel speed latched atthe beginning of the current skid cycle and of the wheel decelerationrate and elapsed time in the current cycle.

According to one aspect of the invention, an anti-skid brake controlsystem for an automotive vehicle comprises a hydraulic brake systemincluding a pressure control valve for adjusting braking pressure at avehicle wheel through at least one skid cycle including a first mode inwhich braking pressure is increased and a second mode in which brakingpressure is decreased, a wheel speed sensor monitoring rotation speed ofthe vehicle wheel and producing a sensor signal, a controller forcontrolling the pressure control valve through the skid cycle so as toprevent skidding of the wheel by adjusting wheel slippage to apredetermined level, the controller monitoring braking conditions andlatching the sensor signal value in response to braking conditionssatisfying a predetermined condition, the controller holding a firstlatched value which is latched at the first occurrence of brakingconditions satisfying the predetermined condition and updating a secondlatched value at each of subsequent occurrence of braking conditionssatisfying the predetermined condition, and the controller deriving acriterion for switching between the first and second modes on the basisof the difference between the first and second latched values and alatching interval of time between the corresponding occurrences.

According to another aspect of the invention, an anti-skid brake controlsystem for an automotive brake system comprises a hydraulic brake systemincluding a wheel cylinder for applying braking pressure to a vehiclewheel, a pressure control valve associated with the wheel cylinder foradjusting fluid pressure in the wheel cylinder, the pressure controlvalve increasing fluid pressure in the wheel cylinder in its first modeof operation and decreasing fluid pressure in the wheel cylinder in itssecond mode of operation, a wheel speed sensor detecting rotation speedof the wheel and producing a wheel speed indicative signal having avalue indicative of the detected wheel speed, and a controller derivinga wheel acceleration value on the basis of variations in the wheel speedindicative signal, comparing the wheel acceleration value with apredetermined deceleration threshold value and latching the wheel speedindicative signal value whenever the wheel acceleration drops below thedeceleration threshold, the controller holding a first latched valuewhich is latched at the first occurrence of wheel acceleration droppingbelow the deceleration threshold, and updates a second latched valuewhich is updated each time the wheel speed indicative signal value islatched, the controller measuring the time interval between the firstoccurrence of latching the wheel speed indicative signal value andsubsequent occurrences deriving a reference value on the basis of thefirst and second latched values and the time interval therebetween,comparing the wheel speed indicative signal value with the referencevalue and producing a control signal ordering the pressure control valveto the second mode when the wheel speed indicative signal valuedecreases to a level below the reference value.

According to a further aspect of the invention, a method for anti-skidcontrolling an automotive brake system comprises the steps of:

producing a wheel speed indicative signal having a value indicative ofthe wheel speed;

deriving brake control parameters on the basis of the wheel speedindicative signal value;

comparing the wheel speed indicative signal value with a first referencevalue and decreasing braking pressure applied to vehicle wheel when thewheel speed indicative signal value assumes a predetermined specificrelationship with the first reference value;

comparing the wheel speed indicative signal value with a secondreference value and increasing braking pressure at the vehicle wheelwhen the wheel speed indicative signal value assumes a predeterminedspecific relationship with the second reference value;

detecting braking conditions on the basis of the brake controlparameter;

latching the wheel speed indicative signal value when braking conditionssatisfy a predetermined condition, holding a first latched value whichis latched the first time braking conditions satisfy the predeterminedcondition and updating a second latched value with the wheel speedindicative signal value each time the predetermined condition issatisfied;

measuring time elapsed following the first time braking conditionsatisfy the predetermined condition;

updating the second reference value with the second latched value eachtime braking conditions satisfy the predetermined condition anddecreasing the second reference value at a rate derived from thedifference between the first latched value and the second latched valueand the elapsed time between the first time and the most time thatbraking conditions satisfied the predetermined condition, and timeelapsed since the most recent time; and

adjusting the first reference value in a predetermined relationship withthe second reference value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of thepreferred embodiments of the present invention, which, however, shouldnot be taken to limit the invention to the specific embodiments, but arefor explanation and understanding only.

FIG. 1 is a schematic block diagram of the overall design of thepreferred embodiment of an anti-skid brake control system according tothe present invention;

FIG. 2 is a perspective view of the hydraulic circuits of the anti-skidbrake system according to the present invention;

FIG. 3 is a circuit diagram of the hydraulic circuits performing theanti-skid control according to the present invention;

FIG. 4 is an illustration of the operation of an electromagnetic flowcontrol valve employed in the hydraulic circuit, which valve has beenshown in an application mode for increasing the fluid pressure in awheel cylinder;

FIG. 5 is a view similar to FIG. 4 but of the valve in a hold mode inwhich the fluid pressure in the wheel cylinder is held at asubstantially constant value;

FIG. 6 is a view similar to FIG. 4 but of the valve in a release mode inwhich the fluid pressure in the wheel cylinder is reduced;

FIG. 7 is a perspective view of a wheel speed sensor adapted to detectthe speed of a front wheel;

FIG. 8 is a side elevation of a wheel speed sensor adapted to detect thespeed of a rear wheel;

FIG. 9 is an explanatory illustration of the wheel speed sensors ofFIGS. 7 and 8;

FIG. 10 shows the waveform of an alternating current sensor signalproduced by the wheel speed sensor;

FIG. 11 is a block diagram of the first embodiment of a controller unitin the anti-skid brake control system according to the presentinvention;

FIG. 12 is a circuit diagram of a projecting speed deriving circuit inthe first embodiment of the controller unit of FIG. 11;

FIG. 13 is a timing chart for the comparator 214 in the controller unitof FIG. 11;

FIG. 14 is a block diagram of another embodiment embodiment of acontroller unit in the anti-skid brake control system according to thepresent invention;

FIG. 15 is a flowchart of a main program of a microcomputer constitutingthe controller unit of FIG. 14;

FIG. 16 is a flowchart of an interrupt program executed by thecontroller unit;

FIG. 17 is a flowchart of a main routine in the main program of FIG. 15;

FIG. 18 is an explanatory diagram of the input timing sampling modes andvariations thereof;

FIG. 19 is a flowchart of an output calculation program for deriving EVand AV signals for controlling operation mode of the electromagneticvalve according to the valve conditions of FIGS. 4, 5 and 6;

FIGS. 20 and 21 are diagrams of execution timing of the outputcalculation program in relation to the main program of FIG. 15;

FIG. 22 is a table for determining the operation mode of the actuator16, which table is accessed in terms of the wheel acceleration and theslip rate; and

FIG. 23 is a flowchart of the projected speed deriving routine in theoutput calculation program of FIG. 19.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, particularly to FIGS. 1 to 10, an anti-skidbrake control system, according to the present invention, includesindependently operative three anti-skid control circuits 402, 404 and406 respectively controlling front-left (FL) wheel, front-right (FR)wheel and rear (R) wheels. The anti-skid control circuit 402, 404 and406 respectively includes digital controller units 202, 204 and 206which are housed in a common controller housing to form a control module200.

The controller unit 202 provided in the front-left anti-skid controlcircuit 402, is connected to a wheel speed sensor 10 for producing analternative current form sensor signal having a frequency proportionalto the rotation speed of a front-left wheel (not shown). On the otherhand, the controller unit 202 is also connected to an electromagneticactuator 16 in a front-left brake circuit 302. The brake circuit 302includes a front-left wheel cylinder 30a for operating a brake shoeassembly 30 for applying braking force to a brake disc rotor 28, and anelectromagnetic pressure control valve 16a operated by the actuator 16for controlling fluid pressure to be applied to the wheel cylinder 30aand whereby controlling the braking force.

Similarly, the controller unit 204 of the front-right anti-skid controlcircuit 404 is connected to a wheel speed sensor 204 to receivealternative current form sensor signal with a frequency representativeof the rotation speed of the front-right wheel. The controller unit 204is, in turn, connected to an actuator 18 in a front-right brake circuit304. The actuator 18 is adapted to operate an electromagnetic pressurecontrol valve 18a for controlling hydraulic pressure to be applied to afront-right wheel cylinder 34a. With the controlled hydraulic pressure,the wheel cylinder 34a operates a front-right brake shoe assembly 34 forapplying braking force to a brake disc rotor 32 rotating with thefront-right wheel.

In addition, the controller unit 206 is connected to a wheel speedsensor 14 to receive therefrom an alternative current sensor signalhaving a frequency indicative of the average rotation speed of rearwheels. In order to detect average rotation speed of the rear wheels,the wheel speed sensor 14 may be adapted to detect rotation speed ofpropeller shaft or the equivalent rotating at the approximately averagespeed of the rear wheels. The controller unit 206 is also connected toan electromagnetic actuator 20 of an electromagnetic pressure controlvalve 20a. The electromagnetic valve 20a is associated with rear wheelcylinders 38a for controlling fluid pressure to be applied to the rearwheel cylinders and whereby controlling braking pressure to be appliedto rear brake disc rotors 36 through rear brake shoe assemblies 38a.

It should be appreciated that through the controller units 202, 204 and206 are adapted to control respectively the front-left, front-right andrear brake circuits 302, 304 and 306, since the embodiment shown isdirected to an anti-skid brake control system for a vehicle having adriving arrangement of a front-engine, rear wheel drive vehicle, theinvention can be modified to apply any driving arrangement of thevehicle, such as front-engine, front wheel drive or four wheel drivearrangements. In addition, though the disclosed brake system comprisesdisc brakes, the anti-skid brake control system according to theinvention can also be applied to drum-type brake system.

The controller units 202, 204 and 206 are respectively associated withactuator drive circuits to control operations of corresponding actuators16, 18 and 20. In addition, each of the controller units 202, 204 and206 is connected to a corresponding wheel speed sensor 10, 12 and 14 viashaping circuits incorporated in the controller 200. Each of the wheelspeed sensors 10, 12 and 14 is adapted to produce an alternating-currentsensor signal having a frequency related to or proportional to therotation speed of the corresponding vehicle wheel. Each of the A-Csensor signals is converted by the corresponding shaping circuit into arectangular pulse signal which will be hereafter referred to as "sensorpulse signal". As can be appreciated, since the frequency of the A-Csensor signals is proportional to the wheel speed, the frequency of thesensor pulse signal should correspond to the wheel rotation speed andthe pulse intervals thereof will be inversely proportional to the wheelrotation speed.

The controller units 202, 204 and 206 operate independently andcontinuously process the sensor pulse signal to derive control signalsfor controlling the fluid pressure in each of the wheel cylinders 30a,34a and 38a in such a way that the slip rate R at each of the vehiclewheels is optimized to shorten the distance required to stop thevehicle, which distance will be hereafter referred to as "brakingdistance".

In general, each controller unit 202, 204 and 206 monitors receipt ofthe corresponding sensor pulses so that it can drive the pulse intervalbetween the times of receipt of successive sensor pulses. Based on thederived pulse interval, the controller units 202, 204 and 206 calculateinstantaneous wheel speed V_(w) and instantaneous wheel acceleration ordeceleration a_(w). From these measured and derived values, a targetwheel speed V_(i) is derived, which is an assumed value derived from thewheel speed at which a slip is assumed to zero or approximately zero.The target wheel speed V_(i) varies at a constant decelerating ratederived from variation of the wheel speed. The target wheel speed thuscorresponds to a vehicle speed which itself is based on variation of thewheel speed. Based on the difference between the instantaneous wheelspeed V_(w) and the target wheel speed V_(i), a slip rate R is derived.The controller units 202, 204 and 206 determine the appropriateoperational mode for increasing, decreasing or holding the hydraulicbrake pressure applied to the wheel cylinders 30a, 34a and 38a. Thecontrol mode in which the brake pressure is increased will be hereafterreferred to as "application mode". The control mode in which the brakepressure is decreased will be hereafter referred to as "release mode".The mode in which the brake pressure is held essentially constant willbe hereafter referred to as "hold mode". The anti-skid control operationconsists of a loop of the application mode, hold mode, release mode andhold mode. This loop is repeated throughout the anti-skid brake controloperation cyclically. One cycle of the loop of the control variationwill be hereafter referred to as "skid cycle".

FIG. 2 shows portions of the hydraulic brake system of an automotivevehicle to which the preferred embodiment of the anti-skid controlsystem is applied. The wheel speed sensors 10 and 12 are respectivelyprovided adjacent the brake disc rotor 28 and 32 for rotation therewithso as to produce sensor signals having frequencies proportional to thewheel rotation speed and variable in accordance with variation of thewheel speed. On the other hand, the wheel speed sensor 14 is providedadjacent a propeller shaft near the differential gear box or drivepinion shaft 116 for rotation therewith (see FIG. 8). Since the rotationspeeds of the left and right rear wheels are free to vary independentlydepending upon driving conditions due to the effect of the differentialgear box 40, the rear wheel speed detected by the rear wheel speedsensor 14 is the average of the speeds of the left and right wheels.Throughout the specification, "rear wheel speed" will mean the averagerotation speed of the left and right rear wheels.

As shown in FIG. 2, the actuator unit 300 is connected to a master wheelcylinder 24 via primary and secondary outlet ports 41 and 43 thereof andvia pressure lines 44 and 42. The master wheel cylinder 24 is, in turn,associated with a brake pedal 22 via a power booster 26 which is adaptedto boost the braking force applied to the brake pedal 22 before applyingsame to the master cylinder. The actuator unit 300 is also connected towheel cylinders 30a, 34a and 38a via brake pressure lines 46, 48 and 50.

The circuit lay-out of the hydraulic brake system circuit will bedescribed in detail below with reference to FIG. 3 which is only anexample of the hydraulic brake system to which the preferred embodimentof the anti-skid control system according to the present invention canbe applied, and so it should be appreciated that it is not intended tolimit the hydraulic system to the embodiment shown. In FIG. 3, thesecondary outlet port 43 is connected to the inlet ports 16b and 18b ofelectromagnetic flow control valves 16a and 18a, the respective outletports 16c and 18c of which are connected to corresponding left and rightwheel cylinders 30a and 34a, via the secondary pressure lines 46 and 48.The primary outlet port 41 is connected to the inlet port 20b of theelectromagnetic valve 20a, the outlet port 20c of which is connected tothe rear wheel cylinders 38a, via a primary pressure line 50. Theelectromagnetic valves 16a, 18a and 20a also have drain ports 16d, 18dand 20d. The drain ports 16d and 18d are connected to the inlet port 72aof a fluid pump 90 via drain passages 80, 82 and 78. The fluid pump 90is associated with an electric motor 88 to be driven by the latter whichis, in turn, connected to a motor relay 92, the duty cycle of which iscontrolled by means of a control signal from the control module 200.While the motor relay 92 is energized to be turned ON, the motor 88 isin operation to drive the fluid pump 90. The drain port 20d of theelectromagnetic flow control valve 20a is connected to the inlet port58a of the fluid pump 90 via a drain passage 64.

The outlet ports 72b and 58b are respectively connected to the pressurelines 42 and 44 via a return passages 72c and 58c. The outlet ports 16c,18c and 20c of respective electromagnetic flow control valves 16a, 18aand 20a are connected to corresponding wheel cylinders 30a, 34a and 38avia braking lines 46, 48 and 50. Bypass passages 96 and 98 are providedto connect the braking pressure lines 46 and 48, and 50 respectively tothe pressure lines 42 and 44, bypassing the electromagnetic flow controlvalves.

Pump pressure check valves 52 and 66 are installed in the pressure lines42 and 44. Each of the pump pressure check valves 66 and 52 is adaptedto prevent the working fluid pressurized by the fluid pump 90 fromtransmitting pressure surges to the master cylinder 24. Since the fluidpump 90 is designed for quick release of the braking pressure in thebraking pressure lines 46, 48 and 50 and thus releasing the wheelcylinders 30a, 34a and 38a from the braking pressure, it is driven uponrelease of the brake pedal. This would result in pressure surges in theworking fluid from the fluid pump 90 to the master cylinder 24 if thepump pressure check valves 66 and 52 were not provided. The pumppressure check valves 66 and 52 serve as one-way check valves allowingfluid flow from the master cylinder 24 to the inlet ports 16b, 18b and20b of the electromagnetic valves 16a, 18a and 20a. Pressureaccumulators 70 and 56 are installed in the pressure lines 42 and 44,which pressure accumulators serve to accumulate fluid pressure generatedat the outlet ports 72b and 58b of the fluid pump 90 while the inletports 16b, 18b and 20b are closed. Toward this end, the pressureaccumulators 70 and 56 are connected to the outlet ports 72b and 58b ofthe fluid pump 90 via the return passages 72c and 58c. Outlet valves 68and 54 are one-way check valves allowing one-way fluid communicationfrom the fluid pump to the pressure accumulators. These outlet valves 68and 54 are effective for preventing the pressure accumulated in thepressure accumulators 70 and 56 from surging to the fluid pump when thepump is deactivated. In addition, the outlet valves 68 and 54 are alsoeffective to prevent the pressurized fluid flowing through the pressurelines 42 and 44 from flowing into the fluid pump 90 through the returnpassages 72c and 58c.

Inlet check valves 74 and 60 are inserted in the drain passages 78 and64 for preventing surge flow of the pressurized fluid in the fluid pump90 to the electromagnetic flow control valves 16a, 18a and 20a after thebraking pressure in the wheel cylinders is released. The fluid flowingthrough the drain passages 78 and 64 is temporarily retained in fluidreservoirs 76 and 62 connected to the former.

Bypass check valves 85, 86 and 84 are inserted in the bypass passages 98and 96 for preventing the fluid in the pressure lines 42 and 44 fromflowing to the braking pressure lines 46, 48 and 50 without firstpassing through the electromagnetic flow control valves 16a, 18a and20a. On the other hand, the bypass check valves 85, 86 and 84 areadapted to permit fluid flow from the braking pressure line 46, 48 and50 to the pressure lines 42 and 44 when the master cylinder 24 isreleased and thus the line pressure in the pressure lines 42 and 44becomes less than the pressure in the braking pressure lines 46, 48 and50.

The electromagnetic flow control valves 16a, 18a and 20a arerespectively associated with the actuators 16, 18 and 20 to becontrolled by means of the control signals from the control module 200.The actuators 16, 18 and 20 are all connected to the control module 200via an actuator relay 94, which thus controls the energization anddeenergization of them all. Operation of the electromagnetic valve 16ain cooperation with the actuator 16 will be illustrated with referenceto FIGS. 4, 5 and 6, in particular illustrating the application mode,hold mode and release mode, respectively.

It should be appreciated that the operation of the electromagneticvalves 18a and 20a are substantially the same as that of the valve 16a.Therefore, disclosure of the valve operations of the electromagneticvalves 18a and 20a is omitted in order to avoid unnecessary repetitionand for simplification of the disclosure.

APPLICATION MODE

In this position, the actuator 16 remains deenergized. An anchor of theelectromagnetic valve 16a thus remains in its initial position allowingfluid flow between the inlet port 16b and the outlet port 16c so thatthe pressurized fluid supplied from the master cylinder 24 via thepressure line 42 may flow to the left front wheel cylinder 30a via thebraking pressure line 46. In this valve position, the drain port 16d isclosed to block fluid flow from the pressure line 42 to the drainpassage 78. As a result, the line pressure in the braking pressure line46 is increased in proportion to the magnitude of depression of thebrake pedal 22 and thereby the fluid pressure in the left front wheelcylinder 30a is increased correspondingly.

In this case, when the braking force applied to the brake pedal isreleased, the line pressure in the pressure line 42 drops due to returnof the master cylinder 24 to its initial position. As a result, the linepressure in the braking pressure line 46 becomes higher than that in thepressure line 42 and so opens the bypass valve 85 to permit fluid flowthrough the bypass passage 98 to return the working fluid to the fluidreservoir 24a of the master cylinder 24.

In the preferring construction, the pump pressure check valve 66,normally serving as a one-way check valve for preventing fluid flow fromthe electromagnetic valve 16a to the master cylinder 24, becomeswide-open in response to drop of the line pressure in the pressure linebelow a given pressure. This allows the fluid in the braking pressureline 46 to flow backwards through the electromagnetic valve 16a and thepump pressure check valve 66 to the master cylinder 24 via the pressureline 42. This function of the pump pressure check valve 66 facilitatesfull release of the braking pressure in the wheel cylinder 30a.

For instance, the bypass valve 85 is rated at a given set pressure, e.g.2 kg/cm² and closes when the pressure difference between the pressureline 42 and the braking pressure line 46 drops below the set pressure.As a result, fluid pressure approximating the bypass valve set pressuretends to remain in the braking pressure line 46, preventing the wheelcylinder 30a from returning to the fully released position. In order toavoid this, in the shown embodiment, the one-way check valve function ofthe pump pressure check valve 66 is disabled when the line pressure inthe pressure line 42 drops below a predetermined pressure, e.g. 10kg/cm². When the line pressure in the pressure line 42 drops below thepredetermined pressure, a bias force normally applied to the pumppressure check valve 66 is released, freeing the valve to allow fluidflow from the braking pressure line 46 to the master cylinder 24 via thepressure line 42.

HOLD MODE

In this control mode, a limited first value, e.g. 2A of electric currentserving as the control signal is applied to the actuator 16 to positionthe anchor closer to the actuator 16 than in the previous case. As aresult, the inlet port 16b and the drain port 16d are closed to blockfluid communication between the pressure line 42 and the brakingpressure line 46 and between the braking pressure line and the drainpassage 78. Therefore, the fluid pressure in the braking pressure line46 is held at the level extent at the moment the actuator is operated bythe control signal.

In this case, the fluid pressure applied through the master cylinderflows through the pressure check valve 66 to the pressure accumulator70.

RELEASE MODE

In this control mode, a maximum value, e.g. 5A of electric currentserving as the control signal is applied to the actuator 16 to shift theanchor all the way toward the actuator 16. As a result, the drain port16d is opened to establish fluid communication between the drain port16d and the outlet port 16c. At this time, the fluid pump 90 serves tofacilitate fluid flow from the braking pressure line 46 to the drainpassage 78. The fluid flowing through the drain passage is partlyaccumulated in the fluid reservoir 76 and the remainder flows to thepressure accumulator 70 via the check valves 60 and 54 and the fluidpump 90.

It will be appreciated that, even in this release mode, the fluidpressure in the pressure line 42 remains at a level higher or equal tothat in the braking pressure line 46, so that fluid flow from thebraking pressure line 46 to the pressure line 42 via the bypass passage98 and via the bypass check valve 85 will never occur.

In order to resume the braking pressure in the wheel cylinder (FL) 30aafter once the braking pressure is reduced by positioning theelectromagnetic valve 16a in the release position, the actuator 16 isagain denergized. The electromagnetic valve 16a is thus returns to itsinitial position to allow the fluid flow between the inlet port 16b andthe outlet port 16c so that the pressurized fluid may flow to the leftfront wheel cylinder 30a via the braking pressure line 46. As set forththe drain port 16d is closed to block fluid flow from the pressure line42 to the drain passage 78.

As a result, the pressure accumulator 70 is connected to the left frontwheel cylinder 30a via the electromagnetic valve 16a and the brakingpressure line 46. The pressurized fluid in the pressure accumulator 70is thus supplied to the wheel cylinder 30a so as to resume the fluidpressure in the wheel cylinder 30a.

At this time, as the pressure accumulator 70 is connected to the fluidreservoir 76 via the check valves 60 and 54 which allow fluid flow fromthe fluid reservoir to the pressure accumulator, the extra amount ofpressurized fluid may be supplied from the fluid reservoir.

The design of the wheel speed sensors 10, 12 and 14 employed in thepreferred embodiment of the anti-skid control system will be describedin detail with reference to FIGS. 7 to 9.

FIG. 7 shows the structure of the wheel speed sensor 10 for detectingthe rate of rotation of the left front wheel. The wheel speed sensor 10generally comprises a sensor rotor 104 adapted to rotate with thevehicle wheel, and a sensor assembly 102 fixedly secured to the shimportion 106 of the knuckle spindle 108. The sensor rotor 104 is fixedlysecured to a wheel hub 109 for rotation with the vehicle wheel.

As shown in FIG. 9, the sensor rotor 104 is formed with a plurality ofsensor teeth 120 at regular angular intervals. The width of the teeth120 and the grooves 122 therebetween are equal in the shown embodimentand define a unit angle of wheel rotation. The sensor assembly 102comprises a magnetic core 124 aligned with its north pole (N) near thesensor rotor 104 and its south pole (S) distal from the sensor rotor. Ametal element 125 with a smaller diameter section 125a is attached tothe end of the magnetic core 124 nearer the sensor rotor. The free endof the metal element 125 faces the sensor teeth 120. An electromagneticcoil 126 encircles the smaller diameter section 125a of the metalelement. The electromagnetic coil 126 is adapted to detect variations inthe magnetic field generated by the magnetic core 124 to produce analternating-current sensor signal as shown in FIG. 10. That is, themetal element and the magnetic core 124 form a kind of proximity switchwhich adjusts the magnitude of the magnetic field depending upon thedistance between the free end of the metal element 125 and the sensorrotor surface. Thus, the intensity of the magnetic field fluctuates inrelation to the passage of the sensor teeth 120 and accordingly inrelation to the angular velocity of the wheel.

It should be appreciated that the wheel speed sensor 12 for the rightfront wheel has the substantially the same structure as the set forthabove. Therefore, explanation of the structure of the right wheel speedsensor 12 will be omitted in order to avoid unnecessary repetition inthe disclosure and in order to simplify the description.

FIG. 8 shows the structure of the rear wheel speed sensor 14. As withthe aforementioned left front wheel speed sensor 10, the rear wheelspeed sensor 14 comprises a sensor rotor 112 and a sensor assembly 102.The sensor rotor 112 is associated with a companion flange 114 which is,in turn, rigidly secured to a drive shaft 116 for rotation therewith.Thus, the sensor rotor 112 rotates with the drive shaft 116. The sensorassembly 102 is fixed to a final drive housing or a differential gearbox (not shown).

Each of the sensor assemblies applied to the left and right front wheelspeed sensors and the rear wheel sensor is adapted to output analternating-current sensor signal having a frequency proportional to orcorresponding to the rotational speed of the corresponding vehiclewheel. The electromagnetic coil 126 of each of the sensor assemblies 102is connected to the control module 200 to supply the sensor signalsthereto.

As set forth above, the control module 200 comprises the controller unit(FL) 202, the controller unit (FR) 204 and the controller unit (R) 206,each of which comprises a microcomputer. Therefore, the wheel speedsensors 10, 12 and 14 are connected to corresponding controller units202, 204 and 206 and send their sensor signals thereto. Since thestructure and operation of each of the controller units is substantiallythe same as that of the others, the structure and operation of only thecontroller unit 202 for performing the anti-skid brake control for thefront left wheel cylinder will be explained in detail.

FIG. 11 shows the first embodiment of controller unit 202 of the presentinvention. The controller units 204 and 206 are designed insubstantially the same way as the controller unit described herebelow.Therefore, in order to simplify the disclosure, the detailed explanationof the controller units 204 and 206 will be omitted.

In FIG. 11, a wheel speed deriving circuit 210 is connected to the wheelspeed sensor 10 to receive wheel speed indicative signals. The wheelspeed deriving circuit 210 is adapted to output a wheel speed indicativesignal having a value proportional to the pulse frequency of the wheelspeed sensor signal from the wheel speed sensor. The wheel speedindicative signal is supplied to an acceleration deriving circuit 212.The wheel acceleration deriving circuit 212 differentiates the wheelspeed indicative signal value to derive wheel acceleration value a_(w)and outputs a wheel acceleration indicative signal. The wheelacceleration indicative signal is input to the negative input terminalof a differential amplifier 214. The positive input terminal of thedifferential amplifier 214 is connected to a reference signal generator216 to receive a reference signal. The reference signal value isrepresentative of a preset deceleration value, e.g. -1G. Therefore, aslong as the wheel acceleration indicative signal value is greater thanthe present deceleration value, the output level of the differentialamplifier 214 remains LOW. On the other hand, when the wheelacceleration indicative signal value becomes less than the presetdeceleration value, output level of the differential amplifier 214 goesHIGH. The output of the differential amplifier 214 is supplied to one ofthe three input terminals of an OR gate 218.

The wheel speed deriving circuit 210 is also connected to a projectedspeed deriving circuit 220. The projected speed driving circuit is alsoconnected to the output of a differential amplifier 219. The negativeinput of the differential amplifier 219 is connected to the wheelacceleration deriving circuit 212. On the other hand, the positive inputof the differential amplifier 219 is connected to a reference signalgenerator 221 which outputs a reference signal having the presetdeceleration value, e.g. -1G. The projected speed deriving circuit 220latches the wheel speed indicative signal value when wheel accelerationindicative signal value becomes equal to or greater than the presetdeceleration value, i.e., in response to a HIGH-level output from thedifferential amplifier 219. The projected speed deriving circuit 220includes memories for storing the latched wheel speed indicative signalvalues of the current skid cycle and the initial wheel speed indicativesignal value which was latched in the first skid cycle. In addition, theprojected speed deriving circuit measures the interval betweenoccurrences of latching of the wheel speed indicative signal value inthe current skid cycle and in the first skid cycle, and from themeasured period of time, the projected speed deriving circuit derives anaverage angular deceleration value. This deceleration value may be usedto derive a projected speed value for the next cycle of anti-skidcontrol. For instance, each instantaneous projected speed may be derivedby the following equation:

    V.sub.c =V.sub.wnew +dV.sub.c ×t

where V_(c) is the projected speed;

V_(wnew) is the wheel speed at which the wheel acceleration indicativesignal value equal to or less than the preset deceleration value isobtained;

dV_(c) is the derived deceleration value; and

t is elapsed time since derivation of the value V_(wnew).

The projected speed V_(c) represents an estimated vehicle speed based onthe measured wheel speed. The vehicle speed can be obtained directlyfrom the wheel speed whenever zero slip can be assumed. Therefore, inthe shown embodiment, it is assumed that, when the preset decelerationvalue, e.g. -1G, is obtained, wheel slip relative to the vehicle groundspeed will be zero or negligible and so can be ignored. The timing atwhich the wheel acceleration value becomes equal to or less than thepreset deceleration value is thus regarded as cripping point forincreasing wheel slippage relative to vehicle from zero by furtherdecelerating operation.

In addition, it should be appreciated that, in the first cycle ofanti-skid control, a fixed value, e.g. -0.4G will be used as thedeceleration value.

Procedures for deriving the projected speed can also be seen in the U.S.Pat. No. 4,392,202, issued July 5, 1983; U.S. Pat. No. 4,384,330, issuedMay 17, 1983; and U.S. Pat. No. 4,430,710 issued Feb. 7, 1984,respectively to the inventor of this invention and commonly assigned tothe assignee of this invention. Disclosure of the above-identified U.S.Patents are herewith incorporated by reference for the sake ofdisclosure.

Returning to FIG. 11, the projected speed deriving circuit 220 isconnected to a target wheel speed deriving circuit 222. The target wheelspeed deriving circuit 222 is adapted to derive a target wheel speedwhich is optimally related to the vehicle speed. The target wheel speedmeans a wheel speed to which the wheel speed is to be adjusted in orderto obtain optimal braking characteristics. In general, as is well known,braking characteristics are optimized when wheel slippage relative tothe vehicle speed is in the range of 15% to 20%. Therefore, according tothe preferred embodiment of the invention, the target wheel speed ischosen to be 85% of the projected vehicle speed. The target wheel speedderiving circuit 222 thus outputs a target wheel speed indicative signalhaving a value corresponding to 85% of the projected speed.

The target wheel speed deriving circuit 222 is connected to the positiveinput terminal of a differential amplifier 224. The negative inputterminal of the differential amplifier 224 is connected to the wheelspeed deriving circuit 210. The differential amplifier 224 compares thewheel speed indicative signal value with the target wheel speedindicative signal value and outputs a HIGH-level comparator signal aslong as the wheel speed indicative signal value is less than the targetwheel indicative signal value. On the other hand, the output level ofthe differential amplifier goes LOW when the wheel speed indicativesignal value becomes greater than the target wheel speed indicativesignal value. The output terminal of the differential amplifier 224 isconnected to an AND gate 228 to supply the comparator output thereto.

The wheel acceleration deriving circuit 212 is also connected to thepositive terminal of a differential amplifier 230. The negative inputterminal of the differential amplifier 230 is connected to a referencesignal generator 232. The reference signal generator 232 outputs areference signal having a value representative of a preset accelerationvalue, e.g. 0.6G. The differential amplifier 230 outputs a HIGH-levelsignal when the wheel acceleration indicative signal value is greaterthan the reference signal value, and, conversely, a LOW-level signalwhen the wheel acceleration indicative signal value is less than thereference signal value.

The OR gate 218 is connected to the output terminals of the threedifferential amplifiers 214, 224 and 230. The differential amplifier 214outputs a HIGH-level comparator signal when the wheel accelerationindicative signal value is less than the preset deceleration value. Thepreset deceleration value represents a pressure release threshold.Therefore, a HIGH-level output from comparator 214 indicatesdeceleration of the wheel beyond the pressure release threshold. Thedifferential amplifier 230 outputs a HIGH-level comparator signal whenthe wheel acceleration indicative signal value is greater than thepreset acceleration value. The preset acceleration value isrepresentative of a pressure apply threshold. Therefore, a HIGH-levelsignal from comparator 230 indicates acceleration of the wheel beyondthe pressure apply threshold. On the other hand, the differentialamplifier 224 outputs a HIGH-level comparator signal when the wheelspeed value is less than the target wheel speed value. Therefore, theoutput of the OR gate 218 is HIGH when wheel acceleration is less thanthe preset deceleration value or greater than the preset accelerationvalue, or when the wheel speed is less than the target wheel speed.

The output of the OR gate 218 is supplied to an amplifier 238 which inturn supplies an amplifier output to the inlet valve as inlet signal EV.

The differential amplifier 224 is also connected for output to an ANDgate 228. In addition, the differential amplifier 230 is connected to aninverting input terminal of the AND gate 228. The differential amplifier224 outputs a HIGH-level comparator signal when wheel speed is less thanthe target wheel speed, the differential amplifier 230 outputs aLOW-level comparator signal when the wheel acceleration value is smallerthe preset acceleration value, and the AND gate 228 outputs a HIGH-levelgate signal only when both of these conditions are satisfied. Otherwise,the output level of the AND gate 228 remains LOW. The AND gate isconnected for output to an amplifier 240 which in turn sends anamplifier signal to outlet valve as outlet signal AV.

The AND gate 228 is also connected to a retriggerable timer 242. Theretriggerable timer 242 is responsive to a HIGH-level output from theAND gate 228 to be triggered for a period of time longer than a maximumduration of one cycle of skid control. The retriggerable timer 242 isconnected for output to the base electrode of a switching transistor 502in a motor control circuit 500 which controls operation of the motor 88of the fluid pump 90. The transistor 502 is rendered conductive by theHIGH-level trigger signal from the retriggerable timer 242 to energize arelay coil 504 connected to collector electrode thereof. A relay switch506 is turned ON by energization of the relay coil 504 to close a powersupply circuit for the motor 88. Therefore, the motor 88 is driven forthe period of time for which the retriggerable timer 242 is triggered.

FIG. 12 shows the projected speed V_(c) deriving circuit 220 employed inthe first embodiment of the anti-skid brake control system of FIG. 11.The projected speed deriving circuit F has four sample/hold circuits220-1, 220-2, 220-3 and 220-4. The sample/hold circuit 220-1 isconnected for input from the wheel speed deriving circuit 210 and fromthe differential amplifier 219 through an AND gate 220-5. The AND gate220-5 is connected to the differential amplifier 219 at one inputterminal and to the retriggerable timer 242 through an inverter 220-6 atits other input terminal.

The sample/hold circuit 220-1 is triggered by HIGH-level signals fromthe AND gate 220-5 produced the output of the differential amplifier 219is HIGH and the retriggerable timer output remains LOW. Since theretriggerable timer 242 outputs a HIGH-level signal in response to aHIGH-level output from the differential amplifier 224 and remains HIGHfor the given period which is long enough to be retriggered by aHIGH-level output from the differential amplifier 224 in the next skidcycle, the sample/hold circuit 220-1 is able to sample the wheel speedV_(w) only during the HOLD mode at the initial stage of skid controloperation. Therefore, the sample/hold circuit holds the initial wheelspeed V_(w0) throughout the anti-skid brake control operation.

The sample/hold circuit 220-2 is also connected for input from the wheelspeed deriving circuit 210 and the differential amplifier 219 directly.The sample/hold circuit 220-2 is triggered by the HIGH level output ofthe differential amplifier 219 to latch the wheel indicative signalvalue V_(w). Since the sample/hold circuit 220-2 is triggered every timethe wheel acceleration drops below the wheel deceleration threshold(-b₁) i.e. in response to each HIGH-level differential amplifier output,its value is updated each time it is triggered.

The sample/hold circuit 220-3 is connected for input from a timer 220-7which is, in turn, connected for input from a clock generator 220-8, forcounting the clock pulses therefrom. The sample/hold circuit 220-3 isalso connected for input from the AND gate 220-5 and is triggered byHIGH-level signals therefrom to latch the timer value t as an initialtimer value t₀.

Although the shown embodiment employs the sample/hold circuit 220-3 forholding the initial timer value, it is not always necessary. As analternative which omits the sample/hold circuit 220-3, the timer 220-7may be designed to be reset by the HIGH-level gate signal from the ANDgate 220-5. In this case, when the initial wheel speed V_(w0) islatched, the timer value t will be 0 by force and thus need not be heldfor the target wheel deriving operation set out below.

The sample/hold circuit 220-4 is connected for input from the timer andis triggered by HIGH-level outputs from the differential amplifier 219.The sample/hold circuit 220-4, when triggered, latches the timer valuet. The contents t of the sample/hold circuit 220-4 is updated every timethe output level of the differential amplifier 219 goes HIGH.

The sample/hold circuits 220-1 and 220-2 are connected for output to asubtractor 220-9. The subtractor 220-9 is adapted to receive the holdingvalues V_(w0) and V_(w) from the sample/hold circuits 220-1 and 220-2.The subtractor 220-9 calculates the difference between the values held(V_(w0) -V_(w)) and outputs a wheel speed difference indicative signaldV_(w). On the other hand, the sample/hold circuits 220-3 and 220-4 areconnected for output to a subtractor 220-10. The subtractor 220-10receives the holding values t₀ and t of the sample/hold circuits 220-3and 220-4. The subtractor 220-10 calculates the difference between theirvalues (t₀ -t) and outputs an elapsed time indicative signal d_(t).

The subtractors 220-9 and 220-10 are connected for output to a divider220-11. The divider 220-11 calculates the rate of wheel speed change(d_(vw) /d_(t)) and produces a deceleration rate indicative signaldV_(c) which is representative of decrement to the projected speed inthe next skid cycle.

The divider 220-11 is connected for output to a multiplier 220-12through a switching circuit 220-13. The switching circuit 220-13 is alsoconnected to an initial deceleration value generator 220-14. The initialdeceleration value generator 220-14 outputs a constant signalrepresentative of a fixed deceleration rate for use in the first skidcycle. On the other hand, the switching circuit 220-13 is connected forcontrol input from a flip-flop 220-15. The flip-flop 220-15 has a setinput terminal connected to the output terminal of an AND gate 220-16.One input terminal of the AND gate 220-16 is connected to thedifferential amplifier 219, and the other input terminal of the AND gate220-16, which is an inverting input terminal, is connected to theretriggerable timer 242. Therefore, similarly to the AND gate 220-5, theAND gate 220-16 outputs a HIGH-level signal only at the beginning of theanti-skid brake control operation. The flip-flop 220-15 also has a resetinput terminal connected to the retriggerable timer 242. The flip-flop220-15 is thus set by HIGH-level signals from the AND gate 220-16 tooutput a HIGH-level switching signal to the switching circuit 220-13.The switching circuit 220-13 is responsive to HIGH-level switchingsignals to connect the initial deceleration value generator 220-14 tothe multiplier 220-12. On the other hand, the flip-flop 220-15 is resetby HIGH-level signals from the retriggerable timer 242 to output aLOW-level switching signal to the switching circuit. While the switchingsignal from the flip-flop 220-15 remains LOW, the switching circuit220-13 connects the divider 220-11 to the multiplier 220-12.

The multiplier 220-12 is also connected for input from to a subtractor220-17. The subtractor 220-17 is connected for input from the timer220-7 and the sample/hold circuit 220-4. The subtractor 220-17 outputsan elapsed period indicative signal d_(p) representative of the timeelapsed since the beginning of the current skid cycle. The multiplier220-12 multiplies the deceleration rate indicative signal value dV_(c)either from the divider 220-11 or the initial deceleraton generator220-14 by the elapsed period indicative signal d_(p) to derive a wheelspeed deceleration value. The multiplier 220-12 outputs a projecteddeceleration indicative signal V_(c') to a subtractor 220-18 which isalso connected to the sample/hold circuit 220-2 to receive the currentvehicle speed value. The subtractor 220-18 subtracts the two inputvalues to derive the instantaneous projected speed V_(c) =V_(w) -V_(c').

The subtractor 220-18 outputs the projected speed indicative signal tothe target wheel speed V_(c) deriving circuit 222.

The operation of the projected speed deriving circuit is shown in FIG.13 in the form of a timing chart.

As shown in FIGS. 11, 12 and 13, at the time, t₂₀, t₂₁ . . . t₂₆ . . . ,the wheel acceleration a_(w) drops below the deceleration threshold(-b₁) to trigger HIGH-level differential amplifier outputs from thedifferential amplifiers 219. At the same time, the output of thedifferential amplifier 214 goes HIGH. This induces HOLD mode operationby which the braking pressure at the wheel cylinders is held at anincreased constant value. At the time t₂₀, the sample/hold circuits220-1 and 220-2 latch the wheel speed indicative signal V_(w) from thewheel speed deriving circuit 202. When the wheel speed V_(w) drops belowthe target wheel speed V_(i) and so triggers RELEASE mode operation, theretriggerable timer 242 becomes active to output a HIGH-level signal.This disables the sample/hold circuit 220-1. Therefore, from this timeon the sample/hold circuit 220-1 holds the initial wheel speed V_(w)sampled at the time t₂₀. On the other hand, the value of the sample/holdcircuit 220-2 is updated every time the output of the differentialamplifier 219 goes HIGH, i.e. at times t₂₁, t₂₂, t₂₃, t₂₄ , t₂₅, t₂₆ . .. , at which the instantaneous wheel speed V_(w1), V_(w2), V_(w3),V_(w4), V_(w5), V_(w6) . . . respectively prevail. At the same time, thesample-hold circuits 220-3 and 220-4 sample and hold the timer values.The sample/hold circuit 220-3 holds the timer value at the time t₂₀throughout the skid control operation as set forth above. Thesample/hold circuit 220-4 updates the held value in response to theleading edge of the HIGH-level output of the differential amplifier 219at times t₂₁, t₂₂, t₂₃, t₂₄, t₂₅, t₂₆ . . . . Based on the values of thesample hold circuits 220-1, 220-2, 220-3 and 220-4, projecting speeddeceleration rates dV_(c1), dV_(c2), dV_(c3), dV_(c4), dV_(c5), dV_(c6). . . are derived. The projected speed values V_(c1), V_(c2), V_(c3),V_(c4), V_(c5), V_(c6) . . . are derived by subtracting the decelerationvalues dV_(c1), dV_(c2), dV_(c3), dV_(c4), dV_(c5), dV_(c6) . . . fromthe sampled wheel speed values V_(w1), V_(w2), V_(w3), V_(w4), V_(w5),V_(w6) . . . .

In the first skid cycle between times t₂₀ and t₂₁, the projected speeddeceleration rate dV_(cset) preset by the initial deceleration generator220-14 is used and the initial wheel speed V_(w0) is taken as thestarting value.

The operation of the first embodiment of the anti-skid brake controlsystem will be described herebelow. Upon application of the brakes, thewheels start to decelerate, i.e. the deceleration value increases. Whenthe wheel deceleration value a_(w) drops below the present decelerationvalue b₁, the output level of the differential amplifier 214 goes fromLOW level to HIGH level. The HIGH level comparator signal of thedifferential amplifier 214 is outputted to the OR gate 218.

At this time, as the wheel acceleration value a_(w) is less than thepreset acceleration value a₁, the output level of the differentialamplifier 230 remains LOW. Also, as the wheel speed V_(w) is higher thanthe target wheel speed V_(i), the output level of the differentialamplifier 224 remains LOW. Therefore, the output of the OR gate 218 goesHIGH, thus transmitting a HIGH-level inlet signal EV to the inlet valve16b through the amplifier 238. On the other hand, since the output levelof the AND gate 228 remains LOW, a LOW-level outlet signal AV istransmitted to the outlet valve 16c via the amplifier 240.

In the embodiment shown, the pressure control valve 16a including theinlet valve 16b and the outlet valve 16c operates in differentoperational modes as set out below:

    ______________________________________                                        Operation Mode      EV        AV                                              ______________________________________                                        APPLICATION MODE    LOW       LOW                                             HOLD MODE           HIGH      LOW                                             RELEASE MODE        HIGH      HIGH                                            ______________________________________                                    

Therefore, the pressure control valve 16a is actuated to the HOLD modein which the fluid pressure to be applied to the wheel cylinder is heldat the pressure level. As a result, the wheel continues to decelerateand drops below the target wheel speed V_(i). As a result, the output ofthe differential amplifier 224 goes HIGH. Since the output level of thedifferential amplifier 230 still remains LOW, the input level of the ANDgate 228 from the differential amplifier 230 is HIGH. Thus, the AND gate228 is opened to transmit a HIGH-level outlet signal AV to the outletvalve 16c via the amplifier 240. At this time, the inlet signal EVremains HIGH. Therefore, the operation mode of the pressure controlvalve 16a changes to the RELEASE mode.

In response to the rising edge of the outlet signal AV, theretriggerable timer 242 is triggered to energize the relay coil 504 toturn ON the relay switch 506 to drive the motor 88 of the pressurereduction fluid pump 90. The fluid pump 90 assists reduction of thefluid pressure in the wheel cylinder.

By actuating the pressure control valve 16a to the RELEASE mode, thewheel speed is allowed to increase again. As a result, the wheelacceleration a_(w) gradually increases and exceeds the presetdeceleration value b₁. Whereupon the output level of the differentialamplifier 214 goes LOW. However, in this case, since the output level ofthe differential amplifier 224 still remains HIGH, the output of the ORgate 224 remains HIGH. Therefore, the RELEASE mode is maintained,allowing the wheel speed to increase further towards the vehicle speed.Wheel acceleration a_(w) thus increases until it exceeds the presetacceleration value a₁. As a result, the output level of the differentialamplifier 230 goes HIGH. The OR gate 218 thus outputs a HIGH-level gatesignal. On the other hand, as the input level of the AND gate 228 fromthe differential amplifier 230 goes LOW, the output level of the ANDgate 228 goes LOW. Therefore, operation returns to the HOLD mode inwhich the fluid pressure in the wheel cylinder is held at the currentpressure level. Since the fluid pressure is relatively low, wheel speedcontinues to increase toward the vehicle speed. The wheel speed thusexceeds the target wheel speed V_(i). Therefore, the output level of thedifferential amplifier 224 turns to a LOW level. At this time, since theoutput level of the differential amplifier 230 is HIGH, the OR gate 218still outputs a HIGH-level gate signal.

FIGS. 14 to 24 show another embodiment of the anti-skid brake controlsystem according to the present invention. In this embodiment, thepresent invention is applied to a microprocessor-based digital controlsystem. In order to make the alternating-current wheel sensor signalapplicable to the digital control system, the sensor signal is convertedinto a train of pulses separated by intervals corresponding to orrepresentative of the detected peripheral speed of the wheel. Beforeexplaining the embodiment shown, the theory of anti-skid brake controlby means of the digital control system will be briefly describedhereinbelow for the sake of better understanding of the invention.

The wheel rotation speed V_(w) is calculated in response to each sensorpulse. As is well known, the wheel speed is generally inverselyproportional to the intervals between the sensor pulses, andaccordingly, the wheel speed V_(w) is derived from the interval betweenthe last sensor pulse input time and the current sensor pulse inputtime. A target wheel speed is designated V₁ and the resultant wheelspeed is designated V_(w). In addition, the slip rate is derived fromthe rate of change of the wheel speed and an projected speed V_(v) whichis estimated from the wheel speed at the moment the brakes are appliedbased on the assumption of a continuous, linear deceleration withoutslippage. In general, the target wheel speed V_(i) is derived from thewheel speed of the last skid cycle during which the wheel decelerationvalue was equal to or less than a given value which will be hereafterreferred to as "deceleration threshold a_(ref) ", and the wheel speed ofthe current skid cycle, and by estimating the rate of change of thewheel speed between wheel speeds at which the rate of deceleration isequal to or less than the deceleration threshold. In practice, the firsttarget wheel speed V_(i) is derived based on the projected speed V_(v)which corresponds to a wheel speed at the initial stage of brakingoperation and at which wheel deceleration exceeds a predetermined value,e.g. -1.2G, and a predetermined deceleration value, for example 0.4G.The subsequent target wheel speed V_(i) is derived based on theprojected speeds V_(v) in last two skid cycles. For instance, thedeceleration value of the target wheel speed V_(i) is derived from adifference of the projected speeds V_(v) in the last two skid cycle anda period of time in which wheel speed varies from the first projectedspeed to the next projected speed. Based on the last projected speed andthe deceleration value, the target wheel speed in the current skid cycleis derived.

The acceleration of the wheel is derived based on the input time ofthree successive sensor pulses. Since the interval of the adjacentsensor signal pulses corresponds to the wheel speed, and the wheel speedis a function of the reciprocal of the interval, by comparing adjacentpulse-to-pulse intervals, a value corresponding to the variation ordifference of the wheel speed may be obtained. The resultant may bedivided by the period of time in order to obtain the wheel accelerationat the unit time. Therefore, the acceleration or deceleration of thewheel is derived from the following equation: ##EQU1## where A, B and Care the input times of the sensor pulses in the order given.

On the other hand, the slip rate R is a rate of difference of wheelspeed relative to the vehicle speed which is assumed as substantiallycorresponding to the target wheel speed. Therefore, in the shownembodiment, the target wheel speed V_(i) is taken as variable orparameter indicative of the assumed or projected vehicle speed. The sliprate R can be obtained by dividing a difference between the target wheelspeed V_(i) and the instantaneous wheel speed V_(w) by the target wheelspeed. Therefore, in addition, the slip rate R is derived by solving thefollowing equation: ##EQU2##

Finally, the controller unit 202 determines the control mode, i.e.,release mode, hold mode and application mode from the slip rate R andthe wheel acceleration or deceleration a_(w).

In anti-skid brake control, the braking force applied to the wheelcylinder is to be so adjusted that the peripheral speed of the wheel,i.e. the wheel speed, during braking is held to a given ratio, e.g. 85%to 80% of the vehicle speed. Therefore, the slip rate R has to bemaintained below a given ratio, i.e., 15% to 20%. In the preferredembodiment, the control system controls the braking force so as tomaintain the slip rate at about 15%. Therefore, a reference valueR_(ref) to be compared with the slip rate R is determined at a value at85% of the projected speed V_(v). As will be appreciated, the referencevalue is thus indicative of a slip rate threshold, which will behereafter referred to "slip rate threshold R_(ref) " throughout thespecification and claims, and varies according to variation of thetarget wheel speed.

In practical brake control operation performed by the preferredembodiment of the anti-skid control system according to the presentinvention, the electric current applied to the actuator attains alimited value, e.g., 2A to place the electromagnetic valve 30a in thehold mode as shown in FIG. 5 when the wheel speed remains inbetween thetarget wheel speed V_(i) and the slip rate threshold R_(ref). When theslip rate derived from the target wheel speed V_(i) and the wheel speedV_(w) becomes equal to or greater than the slip rate threshold R_(ref),then the supply current to the actuator 16 is increased to a maximumvalue, e.g. 5A to place the electromagnetic valve in the release mode asshown in FIG. 6. By maintaining the release mode, the wheel speed V_(w)is recovered to the target wheel speed. When the wheel speed is thusrecovered or resumed so that the slip rate R at that wheel speed becomesequal to or less than the slip rate threshold R_(ref), then the supplycurrent to the actuator 16 is dropped to the limited value, e.g. 2A toreturn the electromagnetic valve 30a to the hold mode. By holding thereduced fluid pressure in the wheel cylinder, the wheel speed V_(w) isfurther resumed to the target wheel speed V_(i). When the wheel speedV_(w) is resumed equal to or higher than the target wheel speed V_(i),the supply current is further dropped to zero for placing theelectromagnetic valve in the application mode as shown in FIG. 4. Theelectromagnetic valve 30a is maintained in the application mode untilthe wheel speed is decelerated at a wheel speed at which the wheeldeceleration becomes equal to or slightly more than the decelerationthreshold a_(ref) e.g. -1.2G. At the same time, the projected speedV_(v) is again derived with respect to the wheel speed at which thewheel deceleration a_(w) becomes equal to or slightly greater than thedeceleration threshold a_(ref). From a difference of speed of the lastprojected speed and the instant projected speed and the period of timefrom a time obtaining the last projected speed to a time obtaining theinstant projected speed, a deceleration rate of the target wheel speedV_(i) is derived. Therefore, assuming the last projected speed isV_(v1), the instant projected speed is V_(v2), and the period of time isT_(v), the target wheel speed V_(i) can be obtained from the followingequation:

    V.sub.i =V.sub.v2 -(V.sub.v1 -V.sub.v2)/T.sub.v ×t.sub.e

where t_(e) is an elapsed time from the time at which the instantprojected speed V_(v2) is obtained.

Based on the input timing, deceleration value a_(w) is derived from theforegoing equation (1). In addition, the projected speed V_(v) isestimated as a function of the wheel speed V_(w) and rate of changethereof. Based on the instantaneous wheel speeds V_(w1) at which thewheel deceleration is equal to or less than the deceleration thresholda_(ref) and the predetermined fixed value, e.g. 0.4G for the first skidcycle of control operation, the target wheel speed V_(i) is calculated.According to equation (2), the slip rate R is calculated, usingsuccessive wheel speed values V_(w1), V_(w2), V_(w3) . . . asparameters. The derived slip rate R is compared with the slip ratethreshold R_(ref). As the wheel speed V_(w) drops below the projectedspeed V_(v) at the time t₁, the controller unit 202 switches the controlmode from the application mode to the hold mode. Assuming also that theslip rate R exceeds the slip rate threshold at the time t₄, then thecontroller unit 202 switches the control mode to the release mode torelease the fluid pressure at the wheel cylinder.

Upon release of the brake pressure in the wheel cylinder, the wheelspeed V_(w) recovers, i.e. the slip rate R drops until it is less thanthe slip rate threshold. The controller unit 202 detects when the sliprate R is less than the slip rate threshold R_(ref) and switches thecontrol mode from release mode to the hold mode.

By maintaining the brake system in the hold mode in which reduced brakepressure is applied to the wheel cylinder, the wheel speed increasesuntil it reaches the projected speed. When the wheel speed V_(w) becomesequal to the target wheel speed V_(i), the controller unit 202 switchesthe control mode from the hold mode to the application mode.

As can be appreciated from the foregoing description, the control modewill tend to cycle through the control modes in the order applicationmode, hold mode, release mode and hold mode. This cycle of variation ofthe control modes will be referred to hereafter as "skid cycle".Practically speaking, there will of course be some hunting and otherminor deviations from the standard skid cycle.

The projected speed V_(v), which is meant to represent ideal vehiclespeed behavior, at time t₁ can be obtained directly from the wheel speedV_(w) at that time since zero slip is assumed. At the same time, thedeceleration value of the vehicle will be assumed to be a predeterminedfixed value or the appropriate one of a family thereof, in order toenable calculation of the target wheel speed for the first skid cycleoperation. Specifically, in the shown example, the projected speed V_(v)at the time t₁ will be derived from the wheel speed V_(w1) at that time.Using the predetermined deceleration value, the projected speed will becalculated at each time the wheel deceleration a_(w) in the applicationmode reaches the deceleration threshold a_(ref).

The wheel deceleration a_(w) becomes equal to or slightly greater thanthe deceleration threshold a_(ref), then the second projected speedV_(v2) is obtained at a value equal to the instantaneous wheel speedV_(w). According to the above-mentioned equation, the deceleration valueda can be obtained

    da=(V.sub.v1 =V.sub.v2)/(t.sub.9 -t.sub.1)

Based on the derived deceleration value da, the target wheel speed V_(i)' for the second skid cycle of control operation is derived by:

    V.sub.i '=V.sub.v2 -da×t.sub.e

Based on the derived target wheel speed, the slip rate threshold R_(ref)for the second cycle of control operation is also derived. The controlmode will be varied during the second cycle of skid control operation,to hold mode, when the wheel deceleration reaches the decelerationthreshold a_(ref) as set forth above, to release mode, when the sliprate R reaches the slip rate threshold R_(ref), to hold mode when theslip rate R recovered to the slip rate threshold R_(ref), and toapplication mode when the wheel speed V_(w) recovered or resumed to thetarget wheel speed V_(i) '. In addition, it should be appreciated thatin the subsequent cycles of the skid control operations, the control ofthe operational mode of the electromagnetic valve as set forth withrespect to the second cycle of control operation, will be repeated.

Relating the above control operations to the structure of FIGS. 3through 6, when application mode is used, no electrical current isapplied to the actuator of the electromagnetic valve 16a so that theinlet port 16b communicates with the outlet port 16c, allowing fluidflow between the pressure passage 42 and the brake pressure line 46. Alimited amount of electrical current (e.g. 2A) is applied as to actuatethe electromagnetic valve 16a to its limited stroke position by means ofthe actuator 16, and the maximum current is applied to the actuator aslong as the wheel speed V_(w) is not less than the projected speed andthe slip rate is greater than the slip rate threshold R_(ref).Therefore, the control mode is switched from the application mode to thehold mode and then to the release mode. The slip rate increases back upto the slip rate threshold R_(ref), so that the control mode returns tothe hold mode, the actuator driving the electromagnetic valve 16a to itscentral holding position with the limited amount of electrical currentas the control signal. When the wheel speed V_(w) finally returns to thelevel of the target speed V_(i), the actuator 16 supply current is cutoff so that the electromagnetic valve 16a returns to its rest positionin order to establish fluid communication between the pressure line 42and the braking pressure line 46 via inlet and outlet ports 16b and 16c.

Refering to FIG. 14, the controller unit 202 includes an input interface1320, CPU 1232, an output interface 1234, RAM 1236 and ROM 1238. Theinput interface 1230 includes an interrupt command generator 1229 whichproduces an interrupt command in response to every sensor pulse. In ROM,a plurality of programs including a main program (FIG. 15), an interruptprogram (FIG. 16), an sample control program, a timer overflow programand an output calculation program (FIG. 19) are stored in respectivelycorresponding address blocks 1244, 1246, 1250, 1252 and 1254.

The input interface also has a temporary register for temporarilyholding input timing for the sensor pulses. RAM 1236 similarly has amemory block holding input timing for the sensor pulses. The contents ofthe memory block 1240 of RAM may be shifted whenever calculations of thepulse interval, wheel speed, wheel acceleration or deceleration, targetwheel speed, slip rate and so fourth are completed. One method ofshifting the contents is known from the corresponding disclosure of theU.S. Pat. No. 4,408,290. The disclosure of the U.S. Pat. No. 4,408,290is hereby incorporated by reference. RAM also has a memory block 1242for holding pulse intervals of the input sensor pulses. The memory block1242 is also adapted to shift the contents thereof according to themanner similar to set forth in the U.S. Pat. No. 4,408,290.

An interrupt flag 1256 is provided in the controller unit 202 forsignally interrupt requests to the CPU. The interrupt flag 1256 is setin response to the interrupt command from the interrupt commandgenerator 1229. A timer overflow interrupt flag 1258 is adapted to setan overflow flag when the measured interval between any pair ofmonitored sensor pulses exceeds the capacity of a clock counter.

In order to time the arrival of the sensor pulses, a clock is connectedto the controller unit 202 to time signals indicative of elapsed realtime. The timer signal value is latched whenever a sensor pulse isreceived and stored in either or both of the temporary register 1231 inthe input interface 1230 and the memory block 1240 of RAM 1236.

The controller unit 202 also includes memory blocks 1235 and 1237 in RAMfor storing the last two wheel acceleration values. The wheel speedindicative signal value V_(w) latched at the first skid cycle will behereafter referred to as "initial wheel speed value V_(w0) " and thewheel speed indicative signal value latched at the beginning of thecurrent skid cycle will be hereafter referred to as "latched wheel speedvalue". The memory block 1235 is allocated for storage of the initialwheel speed value V_(w0). The memory block 1237 stores the latched speedvalue V_(w) and updates the stored value every time a new wheel speedindicative signal value is latched. The controller unit 202 also has atimers 1260 and 1261 and flag registers 1257 and 1258 respectivelyallocated for flags FLCT and FLHD. FLCT is indicative of the operatingstatus of the anti-skid control system. The flag FLHD is indicative ofoperation in the HOLD mode. The timer 1255 measures elapsed time sincethe initial wheel speed indicative signal value was latched. The timer1256 measures the interval over which the hold-indicative flag FLHD isset.

The operation of the controller unit 202 and the function of each of theelement described above will be explained with reference to FIGS. 15 to23.

FIG. 15 illustrates the main program for the anti-skid control system.Practically speaking, this program will generally be executed as abackground job, i.e. it will have a lower priority than most otherprograms under the control of the same processor. Its first step 1002 isto wait until at least one sample period, covering a single sensor pulseor a group thereof, as described in more detail below, is completed asindicated when a sample flag FL has a non-zero value. In subsequent step1004, the sample flag FL is checked for a value greater than one, whichwould indicate the sample period is too short. If this is the case,control passes to a sample control program labelled "1006" in FIG. 15but shown in more detail in FIG. 16. If FL=1, then the control processis according to plan, and control passes to a main routine explainedlater with reference to FIG. 17. Finally, after completion of the mainroutine, a time overflow flag OFL is reset to signify successfulcompletion of another sample processing cycle, and the main programends.

FIG. 16 shows the interrupt program stored in the memory block 1246 ofROM 1238 and executed in response to the interrupt command generated bythe interrupt command generator 1229 whenever a sensor pulse isreceived. It should be noted that a counter value NC of an auxiliarycounter 1233 is initially set to 1, a register N representing thefrequency divider ratio is set at 1, and a counter value M of anauxiliary counter 1235 is set at -1. After starting execution of theinterrupt program, the counter value NC of the auxiliary counter 1233 isdecremented by 1 at a block 3002. The auxiliary counter value NC is thenchecked at a block 3004 for a value greater than zero. For the firstsensor pulse, since the counter value NC is decremented by 1 (1-1=0) atthe block 3002 and thus is zero, the answer of the block 3004 is NO. Inthis case, the clock counter value t is latched in a temporary register1231 in the input interface 1230 at a block 3006. The counter value NCof the auxiliary counter 1233 is thereafter assigned the value N in aregister 1235, which register value N is representative of frequencydividing ratio determined during execution of the main routine explainedlater, at a block 3008. The value M of an auxiliary counter 1235 is thenincremented by 1. The counter value M of the auxiliary counter 1235labels each of a sequence of sample periods covering an increasingnumber of sensor pulses. After this, the sample flag FL is incrementedby 1 at a block 3012. After the block 3012, interrupt program ends,returning control to the main program or back to block 3002, whichevercomes first.

On the other hand, when the counter value NC is non-zero when checked atthe block 3004, this indicates that not all of the pulses for thissample period have been received, and so the interrupt program endsimmediately.

This interrupt routine thus serves to monitor the input timing t of eachpulse sampling period, period, i.e. the time t required to receive NCpulses, and signals completion of each sample period (M=0 through M-10,for example) for the information of the main program.

Before describing the operation in the main routine, the general methodof grouping the sensor pulses into sample periods will be explained tofacilitate understanding of the description of the operation in the mainroutine.

In order to enable the controller unit 202 to accurately calculate thewheel acceleration a_(w), it is necessary that the difference betweenthe pulse intervals of the single or grouped sensor pulses exceeding agiven period of time, e.g. 4 ms. In order to obtain the pulse intervaldifference exceeding the given period of time, 4 ms, which given periodof time will be hereafter referred to as "pulse interval threshold S",some sensor pulses are ignored so that the recorded input timing t ofthe sensor pulse groups can satisfy the following formula:

    dT=(C-B)-(B-A)≧S(4 ms.) (3)

where A, B and C are the input times of three successive sensor pulsegroups.

The controller unit 202 has different sample modes, i.e. MODE 1, MODE 2,MODE 3 and MODE 4 determining the number of sensor pulses in each sampleperiod group. As shown in FIG. 18, in MODE 1 every sensor pulse inputtime is recorded and therefore the register value N is 1. In MODE 2,every other sensor pulse is ignored and the register value N is 2. InMODE 3, every fourth sensor pulse is monitored, i.e. its input time isrecorded, and the register value N is 4. In MODE 4, every eighth sensorpulse is sampled and the register value N is then 8.

The controller unit 202 thus samples the input timing of threesuccessive sensor pulses to calculate the pulse interval difference dTwhile operating in MODE 1. If the derived pulse interval difference isequal to or greater than the pulse interval threshold S, then sensorpulses will continue to be sampled in MODE 1. Otherwise, the inputtiming of every other sensor pulse is sampled in MODE 2 and from thesampled input timing of the next three sensor pulses sampled, the pulseinterval difference dT is calculated to again be compared with the pulseinterval threshold S. If the derived pulse interval difference is equalto or greater than the pulse interval threshold S, we remain in MODE 2.Otherwise, every four sensor pulses are sampled in MODE 3. The inputtimings of the next three sampled sensor pulses are processed to derivethe pulse interval difference dT. The derived pulse interval differencedT is again compared with the pulse interval threshold S. If the derivedpulse interval difference is equal to or greater than the pulse intervalthreshold S, the MODE remains at 3 and the value N is set to 4. On theother hand, if the derived pulse interval difference dT is less than thepulse interval threshold S, the mode is shifted to MODE 4 to sample theinput timing of every eighth sensor pulse. In this MODE 4, the value Nis set at 8.

For instance, in FIG. 18, the sensor pulses A₁, B₁ and C₁ are sampledunder MODE 1. In MODE 2, the sensor pulses a₁ and c₁ are ignored and thesensor pulses A₁ (=A₂), B₂ (=b₁) and C₂ (=b₂ =a₃) are sampled. In MODE3, the three sensor pulses c₂ (=b₃ =a₄), c₃ (=b₄) and c₄ following B₃(=c₂) are ignored and the sensor pulses A₃ (=A₁ =A₂), B₃ (=b₂ =a₃) andC₃ (=b₅ =a₆) are sampled. In MODE 4, the seven sensor pulses c₅ (=b₆=a₇), c₆ (=b₇ =a₈), c₇ (=b₈ =a₉), c₈ (=b₉ =a₁₀), c₉ (=b₁₀ =a.sub. 11),c₁₀ (=b₁₁) and c₁₁ following B₄ (=c₃) are ignored and the sensor pulsesA₄ (=A₁ =A₂ =A₃), B₄ (=C₃ =b₅ =a₆) and C₄ are sampled.

Referring to FIG. 17, the main routine serves to periodically derive anupdated wheel acceleration value a_(w). In general, this is done bysampling greater and larger groups of pulses until the differencebetween the durations of the groups is large enough to yield an accuratevalue. In the main routine, the sample flag FL is reset to zero at ablock 2001. Then the counter value M of the auxiliary counter 1233,indicating the current sample period of the current a_(w) calculationcycle, is read out at a block 2002 to dictate the subsequent programsteps.

Specifically, after the first sample period (M=φ), the input timing ttemporarily stored in the temporary register 1231 corresponding to thesensor pulse number (M=0) is read out and transferred to a memory block240 of RAM at a block 2004, which memory block 1240 will be hereafterreferred to as "input timing memory". Then control passes to the block1008 of the main program. When M=2, the corresponding input timing t isread out from the temporary register 1231 and transferred to the inputtiming memory 1240 at a block 2006. Then, at a block 2008, a pulseinterval Ts between the sensor pulses of M=1 is derived from the twoinput timing values in the input timing memory 1240. That is, the pulseinterval of the sensor pulse (M=1) is derived by:

    Ts=t.sub.1 -t.sub.0

where t₁ is input time of the sensor pulse M1;

and

t₀ is input time of the sensor pulse M0.

The derived pulse interval T_(s) of the sensor pulse M1 is then comparedwith a reference value, e.g. 4 ms., at a block 2010. If the pulseinterval T_(s) is shorter than the reference value, 4 ms., controlpasses to a block 2012 wherein the value N and the pulse interval T_(s)are multiplied by 2. The doubled timing value (2T_(s)) is again comparedwith the reference value by returning to the block 2010. The blocks 2010and 2012 constitute a loop which is repeated until the pulse interval(2nT_(s)) exceeds the reference value. When the pulse interval (2nT_(s))exceeds the reference value at the block 2010, a corresponding value ofN (2N) is automatically selected. This N value represents the number ofpulses to be treated as a single pulse with regard to timing.

After setting the value of N and thus deriving the sensor pulse groupsize then the auxiliary counter value NC is set to 1, at a block 2016.The register value N is then checked for a value of 1, at a block 2018.If N=1, then the auxiliary counter value M is set to 3 at a block 2020and otherwise control returns to the main program. When the registervalue N equals 1, the next sensor pulse, which would normally beignored, will instead be treated as the sensor pulse having the sampleperiod number M=3.

In the processing path for the sample period number M=3, thecorresponding input timing is read from the corresponding address of thetemporary register 231 and transferred to the input timing memory 1240,at a block 2024. The pulse interval T₂ between the sensor pulses at M=1and M=3 is then calculated at a block 2026. The derived pulse intervalT₂ is written in a storage section of a memory block 1242 of RAM 1236for a current pulse interval data, which storage section will behereafter referred at as "first pulse interval storage" and which memoryblock 1242 will be hereafter referred to as "pulse interval memory".After the block 2026, control returns to the main program to await thenext sensor pulse, i.e. the sensor pulse for sample period number M=4.

When the sensor pulse for M=4 is received, the value t of the temporaryregister 1231 is read out and transferred to the input timing memory 240at block 2028. Based on the input timing of the sensor pulses for M=3and M=4, the pulse interval T₃ is calculated at a block 2030. The pulseinterval T₃ derived at the block 2030 is then written into the firstpulse interval storage of the pulse interval memory. At the same time,the pulse interval data T₂ previously stored in the first pulse intervalstorage is transferred to another storage section of the pulse intervalmemory adapted to store previous pulse interval data. This other storagesection will be hereafter referred to as "second pulse intervalstorage". Subsequently, at a block 2030 the contents of the first andsecond storages, i.e. the pulse interval data T₂ and T₃ are read out.Based on the read out pulse interval data T₂ and T₃, a pulse intervaldifference dT is calculated at block 2032 and compared with the pulseinterval threshold S to determine whether or not the pulse intervaldifference dT is large enough for accurate calculation of wheelacceleration or deceleration a_(w). If so, process goes to the block2040 to calculate the wheel acceleration or deceleration according tothe equation (1). The register value N is then set to 1 at the block2044 and thus MODE 1 is selected. In addition sample period number M isreset to -1, and the a_(w) derivation cycle starts again. On the otherhand, if at the block 2032 the pulse interval difference dT is too smallto calculate the wheel acceleration or deceleration a_(w), then thevalue N is multiplied by 2 at a block 2034. Due the updating of thevalue N, the sample mode of the sensor pulses is shifted to the nextmode.

When the block 2034 is performed and thus the sample mode is shifted toMODE 2 with respect to the sensor pulse of M=4', the sensor pulse c₂input following to the sensor pulse of M=4' is ignored. The sensor pulsec₃ following to the ignored sensor pulse c₂ is then taken as the sensorpulse to be sampled as M=3". At this time, the sensor pulse of M=4' istreated as the sensor pulse of M=2" and the sensor pulse of M=2 istreated as the sensor pulse of M=1". Therefore, calculation of theinterval difference dT and discrimination if the derived intervaldifference dT is greater than the pulse interval threshold S in theblock 2032 will be carried out with respect to the sensor pulse c₃ whichwill be treated as the sensor pulse of M=4". The blocks 2032 and 2034are repeated until the interval difference greater than the pulseinterval threshold S is obtained. The procedure taken in each cycle ofrepetition of the blocks 2032 and 2034 is substantially same as that setforth above.

As set forth above, by setting the counter value NC of the auxiliarycounter 233 to 1 at the block 2016, the input timing of the sensor pulsereceived immediately after initially deriving the sample mode at theblocks 2010 and 2012 will be sampled as the first input timing to beused for calculation of the wheel acceleration. This may be constrastedwith the procedure taken in the known art.

FIG. 19 shows the output program for deriving the wheel speed V_(w),wheel acceleration a_(w) and slip rate R, selecting the operationalmode, i.e. application mode, hold mode and release mode and outputtingan inlet signal EV and/or an outlet signal AV depending upon theselected operation mode of the actuator 16.

When the application mode is selected the inlet signal EV goes HIGH andthe outlet signal EV goes HIGH. When the release mode is selected, theinlet signal EV goes LOW and the outlet signal AV also goes LOW. Whenthe selected mode is then hold mode, the inlet signal EV remains HIGHwhile the outlet signal AV goes LOW. These combinations of the inletsignal EV and the outlet signal AV correspond to the actuator supplycurrent levels shown in FIG. 11 and thus actuate the electromagneticvalve to the corresponding positions illustrated in FIGS. 4, 5 and 6.

The output program is stored in the memory block 1254 and adapted to beread out periodically, e.g. every 10 ms, to be executed as an interruptprogram. The output calculation program is executed in the time regionsshown in hatching in FIGS. 20 and 21.

During execution of the output calculation program, the pulse interval Tis read out from a memory block 1241 of RAM which stores the pulseinterval, at a block 5002. As set forth above, since the pulse intervalT is inversely proportional to the wheel rotation speed V_(w), the wheelspeed can be derived by calculating the reciprocal (1/T) of the pulseinterval T. This calculation of the wheel speed V₂ is performed at ablock 5004 in the output program. After the block 5004, the projectedspeed V_(c) is calculated at a block 5005. The projected speedderivation process will be discussed in detail with reference to FIG.23. The target wheel speed V_(i) is calculated on the basis of theprojected speed V_(c) derived at the block 5005, at a block 5006.Derivation of the target wheel speed V_(i) has been illustrated in theU.S. Pat. No. 4,392,202 to Toshiro MATSUDA, issued on July 5, 1983, U.S.Pat. No. 4,384,330 also to Toshiro MATSUDA, issued May 17, 1983 and U.S.Pat. No. 4,430,714 also to Toshiro MATSUDA, issued on Feb. 7, 1984,which are all assigned to the assignee of the present invention. Thecontents of the three United States Patents identified above are herebyincorporated by reference for the sake of disclosure. As is obviousherefrom, the target wheel speed V_(i) is derived as a function of wheelspeed deceleration as actually detected. For instance, the wheel speedV_(w) at which the wheel deceleration a_(w) exceeds the decelerationthreshold a_(ref), e.g., -1.2G is taken as one reference point forderiving the target wheel speed V_(i). The wheel speed at which thewheel deceleration a_(w) also exceeds the deceleration thresholda_(ref), is taken as the other reference point. In addition, the periodof time between the points a and b is measured. Based on the wheel speedV_(w1) and V_(w2) and the measured period P, the deceleration valuedV_(i) is derived from:

    dV.sub.i =(V.sub.w1 -V.sub.w2)/P                           (4)

This target wheel speed V_(i) is used for skid control in the next skidcycle.

It should be appreciated that in the first skid cycle, the target wheelspeed V_(i) cannot be obtained. Therefore, for the first skid cycle, apredetermined fixed value will be used as the target wheel speed V_(i).

At a block 5008, the slip rate R is calculated according to theforegoing formula (2). Subsequently, the operational mode is determinedon the basis of the wheel acceleration a_(w) and the slip rate R, at ablock 5010. The schedule of operation mode selection of the actuator 16is illustrated in the form of a table in FIG. 22. The table is accessedaccording to the wheel acceleration a_(w) and the slip rate R. As can beseen, when the wheel slip rate R is in the range of 0 to 15%, the holdmode is selected when the wheel acceleration a_(w) is less than -1.0Gand the application mode is selected when the wheel acceleration a_(w)is in the range of -1.0G to 0.6G. On the other hand, when the slip rateR remains above 15%, the release mode is selected when the wheelacceleration a_(w) is equal to or less than 0.6G, and the hold mode isselected when the wheel acceleration is in a range of 0.6G to 1.5G. Whenthe wheel acceleration a_(w) is equal to or greater than 1.5G, theapplication mode is selected regardless of the slip rate.

According to the operational mode selected at the block 5010, the signallevels of the inlet signal EV and the outlet signal AV are determined sothat the combination of the signal levels corresponds to the selectedoperation mode of the actuator 16. The determined combination of theinlet signal EV and the outlet signal AV are output to the actuator 16to control the electromagnetic valve.

It should be appreciated that, although the execution timing of theoutput calculation program has been specified to be about 10 ms in theforegoing disclosure, the timing is not necessarily fixed to thementioned timing and may be selectable from the approximate range of 1ms to 20 ms. The execution timing of the output program is fundamentalyto be determined in accordance with the response characteristics of theactuator.

FIG. 23 is a flowchart of a subroutine for deriving the projected speedV_(c). Immediately after starting execution, first, the measured wheelacceleration a_(w) is compared to the deceleration threshold (-b₁) at ablock 5005-1. If the wheel acceleration a_(w) is equal to or less thanthe deceleration threshold (-b₁), the control state indicative flag FLCTin the memory block 1257 is checked at a block 5005-2. If the controlstate indicative flag FLCT is not set when checked at the block 5005-2,and thus the initial stage of anti-skid control is recognized, the HOLDmode indicative flag FLHD in the memory block 1258 is checked at a block5005-3. If the HOLD Mode indicative flag FLHD is not set when checked atthe block 5005-3, the wheel speed indicative signal value V_(w) islatched at a block 5005-4 and is used to preset an initial projectedspeed value V_(c0) and an initial wheel speed value V_(w0) at a block5005-5. Thereafter, the first timer 1255 is started at a block 5005-6.At a block 5005-7, the HOLD mode indicative flag FLHD is set to 1.Thereafter, control returns to the output deriving program of FIG. 19.

On the other hand, if the HOLD mode indicative flag FLHD is set whenchecked at the block 5005-3, a preset initial deceleration rateindicative value dV_(ci), the initial projected speed value V_(c0) andthe timer value t are read from memory at a block 5005-8. It should benoted that the initial deceleration rate indicative value dV_(cset)corresponds to the value indicated by the signal produced by the initialdeceleration value generator in the first embodiment. In the next block5005-9, a projected deceleration value (dV_(c)) is calculated bymultiplying the preset value dV_(ci) by the timer value t. The projecteddeceleration value dV_(c) is then used in block 5005-10 to derive thecurrent projected speed value V_(c), using the formula . . . V_(c)=V_(c0) -dV_(c). 3Control then returns to the main program.

If the control state indicative flag FLCT is set when checked at theblock 5005-2, then the HOLD mode indicative flag FLHD is checked at ablock 5005-11. If the HOLD mode indicative flag FLHD is not set aschecked in the block 5005-11, then the wheel speed indicative signalvalue V_(w) is latched at a block 5005-12. The newly latched speed V_(w)is used to update the initial projected speed value V_(c0) in subsequentblock 5005-13. The timer signal value t and initial vehicle speed valueV_(w0) are then read from memory at a block 5005-14. The timer value tis used to update an initial timer value register t₀ at a block 5005-15.In subsequent block 5005-16, an initial wheel speed deceleration ratevalue dV_(c0) is calculated from the formula dV_(co) =(V_(w0) -V_(w))/t.Finally, the HOLD mode indicative flag FLHD is set at a block 5005-17,before control returns to the main program.

If the HOLD mode indicative flag FLHD is set when checked at the block5005-11, then the initial wheel speed deceleration rate dV_(c0), thetimer value t and the initial timer value t_(o) are read memory in ablock 5005-18. These values are then used in block 5005-19 to find theprojected speed deceleration value V_(c) from the formula dV_(c0)×(t-t_(o)). Then the projected speed V_(c) is calculated using theexpression V_(c0) -dV_(c), at a block 5005-20. Thereafter, controlreturns to the main program.

On the other hand, if the measured wheel acceleration a_(w) checked atthe block 5005-1 is greater than the deceleration threshold (-b₁),control goes to a block 5005-21, in which the HOLD Mode indicative flagFLHD is checked. If the HOLD Mode indicative flag FLHD is set, the HOLDMode indicative flag FLHD is reset at a block 5005-22. Another timer t₂is then started at a block 5005-23. Thereafter, the control stateindicative flag FLCT is checked at a block 5005-24. If the control stateindicative flag FLCT is not set when checked at the block 5005-24, thenFLCT is set at a block 5005-25, and control returns to the main program.

If the HOLD mode indicative flag FLHD is not set when checked at theblock 5005-21, then the control state indicative flag FLCT is checked ata block 5005-26. If the control state indicative flag is set whenchecked at either block 5005-24 or block 5005-26, the timer value t₂ ofthe timer t₂ is read at a block 5005-27. The timer value t₂ is thencompared with a control termination threshold t_(cT) at a block 5005-28.If the timer value t₂ is equal to or greater than the controltermination threshold t_(cT), the control state indicative flag FLCT isreset at a block 5005-29. If either case, control returns to the mainprogram.

As set forth above, according to the present invention, fluctuations ofthe projected speed can be successfully suppressed in comparison withconventional techniques. This results in a smoother and more accuratetarget wheel speed value. Therefore, the characteristics of anti-skidbrake control can be significantly improved.

Therefore, the present invention fulfills all of the objects andadvantages sought therefor.

While the invention has been disclosed in terms of the specificembodiments, the invention should be understood to include allmodifications to the shown embodiments and to other embodiments which donot depart from the principle of the invention set out in the appendedclaims.

What is claimed is:
 1. An anti-skid brake control system for anautomotive vehicle comprising:a hydraulic brake system including apressure control valve for adjusting braking pressure at a vehicle wheelthrough at least one skid cycle and responsive to a control signal forswitching between a first mode in which braking pressure is increasedand a second mode in which braking pressure is decreased; a wheel speedsensor monitoring rotation speed of said vehicle wheel and producing asensor signal; a controller for controlling said pressure control valvethrough said skid cycle so as to prevent skidding of the wheel byadjusting wheel slippage to a predetermined level, said controllermonitoring braking conditions and latching a value of said sensor signalin response to braking conditions satisfying a predetermined conditionin which wheel acceleration decreases across a deceleration criterion,said controller holding a first latched value which is latched at thefirst occurrence of a braking conditions satisfying said predeterminedcondition and updating a second latched value at each subsequentoccurrence of braking conditions satisfying said predeterminedcondition, and said controller deriving a second criterion based onvehicle wheel speed to produce said control signal for switching betweensaid first and second modes of said pressure control valve on the basisof the difference between said first and second latched values and alatching interval of time between the corresponding occurrences.
 2. Theanti-skid brake control system as set forth in claim 1, wherein saidcontroller decreases the value of said second criterion at a given ratewhich is derived from said difference between said first and secondlatched values and said latching interval.
 3. The anti-skid brakecontrol system as set forth in claim 1, wherein said controller derivesa projected speed value on the basis of one of said first and secondlatched values, the difference between said first and second latchedvalues and the latching interval, and derives said second criterion inaccordance with said projected speed.
 4. The anti-skid brake controlsystem as set forth in claim 3, wherein said controller uses one of saidfirst and second latched values as an initial value of said projectedspeed and decreases said initial value at a given rate which is derivedon the basis of said difference between said first and second latchedvalues and said latching interval.
 5. The anti-skid brake control systemas set forth in claim 4, wherein said controller sets said secondcriterion at a value which is a predetermined degree lower than saidprojected speed, said degree representing a desired degree of wheelslippage allowed by braking pressure.
 6. The anti-skid brake controlsystem as set forth in claim 1, wherein said skid cycle consists of afirst stage for increasing said braking pressure and a second stage forholding said braking pressure at a constant value in said first mode, athird stage for decreasing said braking pressure and a fourth stage forholding said braking pressure at a constant value in said second mode.7. The anti-skid brake control system as set forth in claim 6, whereinsaid controller derives a wheel acceleration value on the basis ofvariations of said sensor signal values, and orders said pressurecontrol valve to said second stage when said wheel acceleration dropsacross a predetermined deceleration threshold.
 8. The anti-skid brakecontrol system as set forth in claim 7, wherein said controller latchessaid sensor signal value as one of said first and second latched valueswhen said wheel acceleration drops below said deceleration threshold. 9.An anti-skid brake control system for an automotive brake systemcomprising:a hydraulic brake system including a wheel cylinder forapplying braking pressure to a vehicle wheel, a pressure control valveassociated with said wheel cylinder for adjusting fluid pressure in saidwheel cylinder, said pressure control valve increasing fluid pressure insaid wheel cylinder in a first mode of operation and decreasing fluidpressure in said wheel cylinder in a second mode of operation; a wheelspeed sensor detecting rotation speed of the wheel and producing a wheelspeed indicative signal having a value indicative of the detected wheelspeed; and a controller deriving a wheel acceleration value on the basisof variations in the wheel speed indicative signal, comparing said wheelacceleration value with a predetermined deceleration threshold value andlatching said wheel speed indicative signal value whenever said wheelacceleration drops below said deceleration threshold, said controllerholding a first latched value which is latched at the first occurrenceof wheel acceleration dropping below said deceleration threshold, andupdating a second latched value which is updated each time the wheelspeed indicative signal value is latched, said controller measuring thetime interval between the first occurrence of latching the wheel speedindicative signal value and subsequent occurrences deriving a referencevalued on the basis of said first and second latched values and the timeinterval therebetween, comparing said wheel speed indicative signalvalue with said reference value and producing a control signal orderingsaid pressure control valve to said second mode when said wheel speedindicative signal value decreases to a level below said reference value.10. The anti-skid brake control system as set forth in claim 9, whereinsaid controller derives the difference between said first and secondlatched values, a wheel speed deceleration rate based on said differenceand said time interval between said first occurrence and the occurrencecorresponding to said second latched value, a wheel speed decelerationcharacteristic value on the basis of the second latched value and saiddeceleration rate, said reference value being lower than said wheelspeed deceleration characteristic value by a given amount.
 11. Theanti-skid brake control system as set forth in claim 10, wherein saidpressure control valve is operable in a first stage for increasing saidfluid pressure and a second stage for holding said fluid pressure at anincreased constant value during said first mode of operation, and in athird stage for decreasing said fluid pressure and a fourth stage forholding said fluid pressure at a decreased constant value during saidsecond mode of operation.
 12. The anti-skid brake control system as setforth in claim 11, wherein said controller compares said wheelacceleration value with said deceleration threshold and a predeterminedacceleration threshold and compares said wheel speed indicative signalvalue to a wheel deceleration characteristic value and said referencevalue, said controller operates said pressure control valve in such amanner that:when said wheel acceleration value drops below saiddeceleration threshold while said pressure control valve is in saidfirst stage, said controller orders said pressure control valve to saidsecond stage; when said wheel speed indicative signal value drops belowsaid reference value while said pressure control valve is in said secondstage, said controller orders said pressure control valve to said thirdstage; when said wheel acceleration increases beyond said wheelacceleration threshold while said pressure control valve is in saidthird stage, said controller orders said pressure control valve to saidfourth stage; and when said wheel speed indicative signal value hasincreased beyond said wheel deceleration characteristic value andsubsequently drops below said wheel speed deceleration characteristicvalue while said pressure control valve is in said fourth stage, saidcontroller orders said pressure control valve to said first stage. 13.The anti-skid brake control system as set forth in claim 9, wherein saidcontroller stores a predetermined fixed deceleration rate indicativevalue from which said wheel speed deceleration characteristic value forthe first cycle of skid control operation is derived.
 14. A method foranti-skid controlling an automotive brake system comprising the stepsof:producing a wheel speed indicative signal having a value indicativeof the wheel speed; deriving brake control parameters on the basis ofsaid wheel speed indicative signal value; comparing said wheel speedindicative signal value with a first reference value and decreasingbraking pressure applied to a vehicle wheel when said wheel speedindicative signal value assumes a predetermined specific relationshipwith said first reference value; comparing said wheel speed indicativesignal value with a second reference value and increasing brakingpressure at the vehicle wheel when said wheel speed indicative signalvalue assumes a predetermined specific relationship with said secondreference value; detecting braking conditions on the basis of said brakecontrol parameter; latching said wheel speed indicative signal valuewhen braking conditions satisfy a predetermined condition, holding afirst latched value which is latched the first time braking conditionssatisfy said predetermined condition and updating a second latched valuewith said wheel speed indicative signal value each time thepredetermined condition is satisfied; measuring time elapsed followingthe first time braking conditions satisfy said predetermined condition;updating said second reference value with said second latched value eachtime braking conditions satisfy said predetermined condition anddecreasing said second reference value at a rate derived from thedifference between said first latched value and the second latched valueand the elapsed time between the first time and the most recent timethat braking conditions satisfied said predetermined condition, and timeelapsed since said most recent time; and adjusting said first referencevalue in a predetermined relationship with said second reference value.15. The method as set forth in claim 14, in which said step of derivingbrake control parameters includes a step for deriving a wheelacceleration value on the basis of variation of said wheel speedindicative signal value, and said predetermined condition is satisfiedwhen said wheel acceleration value drops below a predetermineddeceleration threshold.
 16. The method as set forth in claim 15, whichfurther comprises the steps of:comparing said wheel acceleration valueto said deceleration threshold and holding the braking pressure at thevehicle wheel at a constant value when said wheel acceleration valuedrops below said deceleration threshold; and comparing said wheelacceleration value to a predetermined acceleration threshold and holdingthe braking pressure at said vehicle wheel at a constant value when saidwheel acceleration value increases beyond said acceleration threshold.17. The method as set forth in claim 14, wherein braking pressure isdecreased when said wheel speed indicative signal value decreases belowsaid first reference value and increases when said wheel speedindicative signal drops below said second reference value after oncehaving exceeded and second reference value.
 18. An anti-skid brakecontrol system for an automotive vehicle comprising:a hydraulic brakesystem including a pressure control valve for adjusting braking pressureat a vehicle wheel through at least one skid cycle including a firstmode in which braking pressure is increased and a second mode in whichbraking pressure is decreased; a wheel speed sensor monitoring rotationspeed of said vehicle wheel and producing a sensor signal; a controllerfor controlling said pressure control valve through said skid cycle soas to prevent skidding of the wheel by adjusting wheel slippage to apredetermined level, said controller deriving wheel speed based onvalues of said sensor signal and comparing wheel speed with a givenwheel speed reference value for producing a control signal for operatingsaid pressure control valve in said second mode when said wheel speeddecreases across said given wheel speed reference value and foroperating said pressure control valve in said first mode when said wheelspeed decreases across said wheel speed reference value after oncehaving exceeded said wheel speed reference value, said controllerfurther deriving a wheel acceleration based on said sensor signal valuesand latching said sensor signal value in response to braking conditionssatisfying a predetermined condition in which said wheel accelerationdecreases across a given acceleration threshold, said controller holdinga first latched value which is latched at the first occurrence ofbraking conditions satisfying said predetermined condition and updatinga second latched value at each of subsequent occurrence of brakingconditions satisfying said predetermined condition, and said controllerderiving said given wheel speed reference value on the basis of thedifference between said first and second latched values and a latchinginterval of time between the first and second occurrences.
 19. Ananti-skid brake control system for an automotive vehicle comprising:ahydraulic brake system including a pressure control valve for adjustingbraking pressure at a vehicle wheel through at least one skid cycle andresponsive to a control signal for switching between a first mode inwhich braking pressure is increased and a second mode in which brakingpressure is decreased; a wheel speed sensor monitoring rotation speed ofsaid vehicle wheel speed and producing a sensor signal; a controller forcontrolling said pressure control valve through said skid cycle so as toprevent skidding of the wheel by adjusting wheel slippage to apredetermined level, said controller responsive to said sensor signalfor monitoring braking conditions and sampling a reference data value ofsaid sensor signal in response to braking condition satisfying apredetermined condition in which wheel acceleration decreases across adeceleration criterion at a predetermined timing, said controllerincluding means for holding said reference data value as a first valueover sequentially occurring skid cycles, said controller also sampling atemporary value at every subsequent occurrence of braking conditionssatisfying said predetermined condition and including means fortemporarily storing said temporary value until the next occurrence ofsaid braking condition satisfying said predetermined condition, as asecond value, and said controller further including means for samplingtime data representative of an elapsed time between the occurrence ofsampling of said reference data value and the occurrence of thecurrently stored temporary value for deriving a second criterion basedon a variation rate of said wheel speed over said interval, saidcontroller responsive to said time data and vehicle wheel speed forswitching between said first and second modes of said pressure controlvalve.